1. Field of the Invention
The present invention relates to an optical coupling between an optical transmission medium and an optical device. More particularly, the present invention relates to a coupling structure and an optical-alignment method of aligning the optical device to a planar lightwave circuit precisely by bonding the optical device to a substrate through a flip-chip bonding process.
2. Description of the Related Art
Due to the recent demand for data communications including audio and video signals in the Internet, the conventional communication system based on an electric signal is replaced with an optical communication system.
The optical communication involves converting an electric signal into light energy and then restoring light energy into the electric signal. To this end, an optical transmitter, an optical-transmission medium, and an optical receiver are utilized. A PLC (planar lightwave circuit) device is used mainly as the optical-transmission medium. An LD (laser diode) or a VCSEL (vertical cavity surface emitting laser) is used mainly as the optical transmitter. A semiconductor optical device, such as a PD (photo diode) is used mainly as the optical receiver.
A flip-chip bonding technique has been used typically in fabricating an electronic circuit requiring a small-size and a high-speed operation. Using the flip-chip bonding technique, the volume of an electronic circuit fabricated may be reduced to about 1/10 of the conventional ceramic-packaged integrated circuit. In addition, a design aspect of the electronic circuit can be simplified and the length of an electric connection wire can be shortened. Thus, it is advantageous for implementation in high-speed operation.
When fabricating an optical transmitter and an optical receiver, optical devices including the LD, VCSEL, and PD are coupled directly to the substrate in such a manner that the optical devices can be connected optically to an optical fiber. Further, the PLC may be aligned on the substrate by adopting a bonding technique similar to the flip-chip bonding technique. When coupling the optical-transmission medium, optical transmitter, and optical receiver optically with each other, coupling efficiency must be considered carefully as the coupling efficiency tends to be a main factor in obtaining an optimal performance of the optical-transmission medium, optical transmitter, and optical receiver. Accordingly, a precise optical alignment is highly desired.
FIGS. 1A to 1D are views showing a conventional alignment method between a VCSEL and a PLC.
As shown in FIG. 1A, a V-groove 3 is formed on a substrate 2 having a VCSEL 1 attached thereto. Then, a protrusion 5 corresponding to the V-groove 3 is formed on a pickup device 4 for picking up the substrate 2. The pickup device 4 picks up the substrate 2 in an arrow direction in such a manner that the protrusion 5 is positioned within the V-groove 3 (referred to FIG. 1B), thereby maintaining the VCSEL 1 and the V-groove 3 in a predetermined position. The substrate 2, which has been picked up by the pickup device 4, is inserted into a guide hole 8 formed in a package or an OSA 7 (optical sub-assembly 7 shown in FIG. 1C) through a guiding pin 6 attached to the pickup device 4, so that the substrate 2 can be attached to a predetermined portion of the OSA 7.
FIGS. 2A and 2B are views showing another conventional alignment method. FIG. 2A shows a guide hole 12 formed on an OSA (Optical Sub-Assembly) structure and a guiding pin 14 of a chip assembly block 13, which are aligned through a passive-alignment method. FIG. 2B is a front view of the chip assembly block 13 shown in FIG. 2A. Reference numerals 15 and 16 represent a PLC and a ferrule, respectively.
Operating the above conventional alignment, however, has the following disadvantages:
Firstly, the pickup device is large and requires a complex structure, and a plurality of pickup devices that corresponds to the sorts of substrates is also required. Next, since the V-groove is formed on an upper surface of the substrate, the size of a pattern or the mounting area formed on the substrate is reduced so that it is difficult to reduce the size of the substrate. Moreover, the guiding pin and the guide hole must be formed in the substrate and the package or the OSA, respectively, in order to mount the substrate on the package or the OSA. As such, it is required to precisely form the guide hole in the package or the OSA that in turn enlarges the size of the package or the OSA. For this reason, although the conventional alignment method can mount the substrate on the package or the OSA by picking up the substrate formed with the V-groove using the guiding pin, the structure of the pickup device, substrate, package, and OSA are complex and undesirably large. Furthermore, an output power band of a transceiver is increased due to a mechanical processing error according to the conventional method, so it is difficult to adjust the transmission characteristics as desired. In addition, since the level of the light received in a light-detecting device is not constant, it is difficult to adjust the sensitivity of the operating device. Lastly, in order to reduce the mechanical processing error in the main components, fabrication cost may be increased, and as a result, the price competitiveness of an article suffers.